The driving forces for fluid (water) movement across the capillary wall are hydrostatic pressure and oncotic pressure (i.e., osmotic pressure generated by proteins). Collectively, these are called the Starling forces. Capillary fluid movement is discussed in detail in in brief, hydrostatic pressure within the capillary (as a result of the pumping of the heart and the effect of gravity on the column of blood in the vessels feeding a capillary) is a force that causes fluid to move out of the capillary. Hydrostatic pressure in the surrounding interstitial tissue opposes the effect of the capillary hydrostatic pressure. The oncotic pressure of the plasma in the capillary tends to draw fluid from the interstitium into the capillary. The oncotic pressure of the interstitial fluid opposes this. Thus the amount of fluid moving across the wall of the capillary is determined as follows:
Equation 2.1
Kf = filtration constant (measure of surface area + in πi = interstitial fluid oncotic pressure.
Depending on the magnitude of these forces, fluid may move out of the capillary or into the capillary.
The compositions of the various body fluid compartments differ; however, as described later, the osmolalities of the fluid within these compartments are essentially identical.
Thus the compartments are in “osmotic equilibrium.” In addition, any change in the osmolality of one compartment quickly causes water to redistribute across all compartments, which brings them back into osmotic equilibrium. Because of this rapid redistribution of water, measuring the osmolality of plasma or serum, which is easy to do, reveals the osmolality of the other body fluid compartments (i.e., interstitial fluid and intracellular fluid).
As described later, Na+ is a major constituent of the ECF. Because of its high concentration in comparison with other molecules and ions, Na+ (and its attendant anions, primarily Cl− and HCO3 −) is the major determinant of the osmolality of this compartment. Accordingly, it is possible to obtain an approximate estimate of the ECF osmolality by simply doubling the sodium concentration [Na+]. For example, if a blood sample is obtained from an individual, and the [Na+] of the serum is 145 mEq/L, its osmolality can be estimated as follows:
Equation 2.2
In contrast to water, the movement of ions across cell membranes is more variable from cell to cell and depends on the presence of specific membrane transport proteins (see the section “
Composition of Body Fluid Compart ”). Consequently, in trying to understand the physiology of fluid shifts between body fluid compartments, it can be assumed that while water moves freely between the compartments, there is little net movement of solutes. For most situations, this is a reasonable assumption.
To illustrate the physiologic characteristics of fluid shifts, consider what happens when solutions containing various amounts of NaCl are added to the ECF.
Example 1: Addition of Isotonic Sodium Chloride to the Extracellular Fluid